You can’t stop people from cracking your software! You simply can’t.
Here is a nice article on why it does not make sense to worry about:
http://blogs.balsamiq.com/product/2008/10/19/my-views-on-software-piracy/
And here about obfuscation:
Protect .NET code from reverse engineering?
You can also check the signature/hashsum, etc of your control assembly in your license.